An example of conventional air bag systems is an air bag system for protecting a driver, which is incorporated in the center pad of a boss portion of the steering wheel of a vehicle This air bag system essentially comprises a base plate fastened to the body of the steering wheel, a basically cylindrical gas generator (generally called an inflator) mounted on the base plate, a pouch-shaped air bag so folded as to cover the inflator, and a cover so attached to the base plate as to cover the air bag. With the structure as above, when the control unit of the system detects a shock which may result from a collision of the vehicle, the air bag system actuates the inflator to cause a large quantity of inert gas to be instantaneously ejected from the inflator into the air bag, thereby unfolding and inflating the air bag. The pressure of the expansion of the air bag tears the cover and projects the air bag towards the front of the driver, where the air bag continues to be unfolded and inflated at the same time to protect the driver by thus reducing the shock received by the driver.
A need exists for the air bag of an air bag system described above to be so constructed and arranged in the system as to project from the cover speedily by tearing the cover instantaneously and to be inflated into a specified shape, such as a wide, flat shape spreading in front of the driver.
An example of structures dealing with the above problems is disclosed in Japanese Patent Laid-open No. 127330/1994, which uses straps having a specified length and sewed at the two ends of each strap to the inner face of the front and the bottom fabric materials of the air bag in order to limit the height (the distance) by which the air bag projects forward, towards the person seated in front of the air bag system (hereinafter simply referred to as a passenger or the passenger) and allow the air bag to smoothly expand in the radial direction (in the direction of the circumferential edge of the air bag). However, the above structure that uses straps bridging the inner surface of the fabric materials necessitates a complicated sewing operation to sew the fabric materials as above and consequently increases production costs. As the straps receive a great load when the air bag is inflated, the straps have to be ensured to have a sufficient strength. This, too, increases production costs. Furthermore, as the straps inside the air bag increase the volume of the whole folded air bag, it is difficult to provide a compact air bag system.
Another example of air bag systems is disclosed in Japanese Patent Laid-open No. 305387/1994. An air bag disclosed therein has a stitched portion where the front fabric material and the back fabric material of the air bag are sewed together at a specified location or locations in the state that the bag is spread out in a circular state. Thereafter, tie air bag is folded up into a compact shape and disposed in a housing. According to the above structure, when gas is ejected into the air bag, the stitched portions limit the forward inflation of the air bag, thereby allowing the air bag to smoothly expand in the radial direction, towards its circumferential edge. Then, at a specified stage during the process of the inflation of the air bag, the threads of the stitched portions are tom so that the air bag expands forward, in the direction of the passenger. However, the structure that calls for folding an air bag after sewing together the front and the back fabric materials presents a problem in that it is difficult to make use of gas pressure effectively to tear the cover, because expansion of the air bag is limited until the threads are broken. Furthermore, as the air bag expands forward when the increased internal pressure in the air bag has broken the threads, it is necessary to conduct thorough research and investigations concerning various factors such as the radial dimension of the air bag, how to fold the air bag and the breaking strength of the stitching thread in order to appropriately control the distance which the air bag is projected forward so as to expand the air bag widely and evenly. This means that reduction of production costs is difficult according to the above structure.
Another example structure wherein the fabric materials of an air bag are sewed together is disclosed in Japanese Patent Laid-open No. 329664/1995, which calls for folding the peripheral edge of an air bag inward and stitching this folded portion at a stitched portion. This structure, however, has a drawback wherein reduction of production costs is difficult because not only is a complicated procedure of turning the air bag inside out necessary but it is also difficult to smooth out the stitched portions to eliminate wrinkles.
Another example is disclosed in Japanese Patent Laid-open No. 277129/1995, which calls for providing breakable ear portions around the circumferential edge of an air bag, bending these ear portions towards the bottom face of the air bag and secure them by means of bolts which fasten the air bag and the inflator. According to this structure, however the ear portions formed at the peripheral edge of the air bag are fastened to the bottom of the system. Therefore, when the air bag is inflated, the ear portions and the portions of the air bag fabric which are rolled together with the ear portions are pushed against the base plate or the inner surface of the cover, preventing the ear portions from breaking smoothly, and consequently making it difficult for the air bag to be unfolded smoothly.
As described above, the above structure that uses straps bridging the inner surface of the fabric material at two locations necessitates a complicated sewing operation to sew the fabric in such a manner and consequently increases production costs. As the straps receive a great load when the air bag is inflated, the straps have to be ensured to have a sufficient strength, which, too, increases production costs, Furthermore, as the provision of the straps inside the air bag increase the volume of the whole folded air bag, the above structure presents another problem in that it is difficult to provide a compact air bag system. The structure that calls for sewing the fabric materials at the front and the back sides of the air bag together and subsequently folding the air bag, too, has a drawback wherein making use of gas pressure effectively to tear the cover is difficult, because expansion of the air bag is limited until the thread is broken. In addition, as the air bag expands forward when the increased internal pressure in the air bag has broken the thread, it is necessary to conduct thorough research and investigations concerning various factors such as the radial dimension of the air bag, how to fold the air bag and the breaking strength of the stitching thread in order to appropriately control the distance which the air bag is projected forward so as to expand the air bag widthwise and evenly, and such criteria makes reduction of production costs difficult. Furthermore, the structure that calls for folding the peripheral edge of an air bag inward and stitching the folded portion at a stitched portion makes it difficult to reduce production costs because not only is a complicated procedure of turning the air bag inside out necessary but it is also difficult to smooth out the stitched portion to eliminate wrinkles According to the structure that calls for providing breakable eat portions at the circumferential edge of an air bag, bending these ear portions towards the bottom of the air bag and fastening them by means of bolts which secure the air bag and the inflator, the ear portions and the portions of the air bag fabric which are rolled together with the ear portions are pushed against the base plate or the inner surface of the cover when the air bag is inflated. As a result the ear portions are prevented from breaking smoothly, and it is therefore difficult for the air bag to be unfolded smoothly.
In order to solve the above problems, an object of the present invention is to provide an air bag system and a method of folding an air bag which are capable of rapidly unfolding and inflating an air bag.